The present invention relates generally to the field of semiconductor lasers, and more specifically to a semiconductor laser coaxially integrated with a phototransistor.
Semiconductor lasers, also referred to as diode lasers, are well known in the art. These devices generally consist of a planar layered semiconductor structure having one or more active layers bounded at their ends by cleaved surfaces that act as mirrors for the optical resonator. In one form of this structure the layers on one side of the active layer or layers are doped with impurities so as to have an excess of mobile electrons, while on the other side of the active layer(s) the semiconductor layers are doped with impurities so as to have a deficiency of mobile electrons, therefore creating an excess of positively charged carriers called holes. Layers with excess electrons are said to be n-type, i.e. negative, while layers with excess holes are said to be p-type, i.e. positive. Activation of the laser is achieved by applying an electrical potential between the p-side and the n-side of the layered structure, thereby driving either holes or electrons or both in a direction perpendicular to the planar layers so as to "inject" them into the active layers, where electrons recombine with holes to produce light. Optical feedback provided by the cleaved mirrors allows resonance of some of the emitted light to produce coherent "lasing".
In another form of diode laser, one region of the active layer or layers in the plane of the active layer is made to be p-type while another region in the plane of the active layer is made to be n-type. A p-n junction is thereby formed in the active layer or layers, so as to drive electrons or holes in a direction parallel to the plane of the active layers. A p-type optical waveguide for the lasing action can be formed by placing n-type regions on either side of the p-type region, thereby forming an n-p-n laser structure, as described by Thornton, et al., in Unified Planar Process for Fabricating Heterojunction Bipolar Transistors and Buried-Heterostructure Lasers Utilizing Impurity-Induced Disordering, Applied Physics Letters, vol. 53, no 26, pp. 2669-2671, (1988). Activation of the p-type waveguide is achieved by applying an electrical potential between the p-side and n-side of both junctions, thereby injecting electrons into the p-type region, where they recombine with holes to produce light. Similarly an n-type optical waveguide for the lasing action can be formed in an n-type active layer or layers, by placing p-type regions on either side of the n-type region, thereby forming a p-n-p laser structure. Activation of the n-type waveguide is achieved by applying an electrical potential between the p-side and n-side of both junctions thereby injecting holes into the n-type region, where they recombine with electrons to produce light. Both of these structures are known as transverse junction lasers because the p-n junction lies across, i.e. transverse to, the active layers.
The transverse n-p-n or p-n-p laser structure described above also functions as a bipolar transistor as disclosed in detail in U.S. Pat. No. 4,987,468, dated Jan. 22, 1991, to Thornton. For example, FIG. 1 shows the n-p-n structure fabricated by impurity-induced layer disordering on a semi-insulating substrate with the base, emitter, and collector contacts for the transistor all placed on the top surface of the semiconductor layers. Since the base region is necessarily kept very narrow, on the order of a few microns in width, the base contact is placed remote from the active base region with a conductive path allowed around and under one of the n-type regions. As with other transistor structures, the bipolar n-p-n device is operated in common-emitter mode by applying a positive voltage between the p-type base and the n-type emitter, thereby forward-biasing the base/emitter junction, and a positive voltage between the n-type collector and the n-type emitter, thereby reverse-biasing the collector base junction.
It is well known in the art that a transistor structure can be used as an optical detector with gain by introducing the incident light into the base region. See, e.g. J. C. Campbell, Phototransistors for Lightwave Communications, in Semiconductors and Semimetals, vol. 22, part D, edited by W. T. Tsang, pp. 389-447, Academic Press, (1985). In the phototransistor, excess electron-hole pairs are created in the base by absorption of the input light. The electrons diffuse to and are collected by the reverse-biased collector junction, leaving excess holes in the p-type base that lower the forward-biased potential on the emitter junction, causing electrons to be injected to the base from the emitter. Because of the exponential dependence of current on junction potential in a forward-biased junction, more electrons are injected into the base than the holes created by the incident light, resulting in a current gain. Thus the current proportional to the incident light power generated in the base is amplified by normal transistor action.
Optical detectors monolithically integrated with diode lasers have been of considerable interest to those skilled in the art, as a way to monitor the level of power emitted by a diode laser, especially for monolithic arrays of independently addressed lasers. One approach, described by Merz, et al., in Integrated GaAs-Al.sub.x Ga.sub.1-x As Injection Lasers and Detectors with Etched Reflectors, Applied Physics Letters vol. 30, No. 10, pp. 530-533, (1977), employs a detector region containing the same layer configuration as the laser, located outside the laser resonator but coaxial with the direction of lasing and coupled to the laser by an underlying passive waveguide layer. A second approach, described by Iga, et al., in GalnAsP/InP Laser with Monolithically Integrated Monitoring Detector, Electronics Letters vol 16, pp. 342-343, (1980), employs a detector region containing the same layer configuration as the laser, separated from the laser by an etched groove. Both of these approaches employ in-line detectors which are outside the laser cavity and require at least one laser facet to be etched. Other novel methods and apparatus employing in-line detectors are disclosed in U.S. Pat. No. 5,136,604 assigned to the assignee hereof. In addition to in-line detection techniques, Kobayashi, in U.S. Pat. No. 4,674,100, dated Jun. 16, 1987, has disclosed the use of a diffraction grating within the laser cavity to deflect light in a direction normal to the plane of the active layer of the laser and into an integrated detector. Other methods and apparatus for deflecting light out of a laser cavity into a detector region are disclosed in the aformentioned U.S. Pat. No. 5,136,604.
One important application for a photodector integrated with a diode laser is in a feedback loop that stabilizes the power emitted by the laser against unpredictable variations. Thermal fluctuations are especially deleterious to maintaining constant optical power output, especially during pulsed modulation. For example, heating of the laser chip unavoidably occurs when the applied laser current is abruptly increased at the beginning of a pulse. Since a laser's output power is temperature dependent, this time-dependent, or transient, heating normally causes the power output to decrease or "droop" during the pulse. Furthermore, transient heating during a sequence of pulses can have a cumulative effect on the temperature that depends on the number and frequency of the pulses. For example, if the time between successive pulses is large, the device will be given sufficient time to cool, so that the application of the driving current has a large temperature effect, i.e. droop, during the next pulse. The shorter the time between pulses, the less time the device has a cool between one pulse and the next, leading to a sustained increase in the temperature of the laser. This sustained temperature increase results in a further decrease in amplitude of the output pulse obtained with a constant level of input current.
There is presently a need in the art for apparatus and methods which provide accurate and reproducible control of the optical energy contained in each pulse of an intensity-modulated light beam. For example, as described in concurrently filed U.S. patent application Ser. No. 07/906,145 and incorporated herein by reference thereto, digital printing on a photosensitive recording medium requires accurate control of the optical energy delivered in each pulse. In systems currently known to those skilled in the art, a predetermined amount of energy is delivered in each pulse by turning on the optical beam to a desired power level for a fixed time interval. This approach requires that the laser output power be reproducible from pulse to pulse and constant during a pulse, in order that the optical energy delivered in each pulse be accurately controlled. Accurate control is especially important in printing with different grey levels formed by varying the number of exposed spots or when exposing very closely spaced spots in order to control the formation of an edge. Power fluctuations can arise from many sources, including for example ambient thermal fluctuations, laser self-heating, laser degradation, fluctuations in the drive current, and/or the pattern of modulation. Since these fluctuations occur within each pulse and are very difficult to predict, control, or eliminate, they are commonly not compensated for. Thus there is presently a need in the art for apparatus and methods which provide accurate and reproducible control of the level of pulsed optical power emitted by an intensity-modulated diode laser.
Each of the integrated detectors of the prior art discussed above is formed by a simple p-n junction diode. A p-n junction diode of this type provides an electrical signal proportional to the laser power but does not provide amplification of the photo-generated current. Consequently such a detector requires an additional stage of amplification to produce feedback control sufficient to maintain constant laser output in the presence of unpredictable variations in laser temperature, drive current, or modulation pattern. Additional amplification can be obtained from an off chip circuit or by monolithically incorporating a remote transistor on the chip, e.g. as described by Katz, et. al., in a Monolithic Integration of GaAs/GaAlAs Bipolar Transistor and Heterostructure Laser, Applied Physics Letters vol. 37, pp. 211-213 (1980). However, instantaneous stabilization of the laser's output power requires high-speed electrical coupling from the detector to the transistor to the laser. Consequently, external circuitry is adequate to compensate slow variations of power but not for instantaneous stabilization during short pulse modulation such as encountered in laser printing systems. Similarly, with on-chip amplification, the required response times dictate that the transistor amplifier be physically located close to the laser/detector structure. Consequently, remote amplifying transistors are not suitable for use with more than two closely spaced lasers on a chip because the transistors must be placed too far from the laser/detector structure to allow adequately fast response. Accordingly, there is presently a need in the art for a compact monolithic apparatus emitting constant optical power either continuously or from pulse to pulse. These and other problems are addressed by various aspects of the present invention, which will be summarized and then described in detail below.